Evidence for an Isomeric 29/2⁺ Level in ²¹¹At

Abstract

A 70 ns isomeric state at 2 641 keV has been produced in the 209 Bi(α,2n) 211 At reaction and interpreted as the 29/2 + state of the h 9/2 2 i 13/2 proton configuration. It decays to the states ( h 9/2 2 f 7/2 ) 23/2 , ( h 9/2 3 ) 21/2 , ( h 9/2 3 ) 17/2 , ( h 9/2 3 ) 13/2 and ( h 9/2 3 ) 9/2 by a straight cascade of transitions with energies (multipolarities) 713.3 keV ( E 3), 511.6 keV ( M 1), 96.0 keV ( E 2), 253.4 keV ( E 2), and 1 066.9 keV ( E 2). There is also indirect evidence for a 24.4 keV ( E 2) branch to a ( h 9/2 2 i 13/2 ) 25/2 state followed by a 688.9 keV ( E 1) transition to the 21/2 - state. The reduced transition rate B ( E 3, 29/2 + → 23/2 - ) = 51 000 e 2 fm 6 can be reproduced by taking the value 0.9 MeV for the particle-phonon coupling matrix element ⟨ i 13/2 , 3 - ; 7/2| H coupl | f 7/2 ⟩.